The radio frequency (RF) spectrum is often segmented or divided into various frequency bands for use with differing types of wireless devices. In order to prevent interference, these devices typically may be licensed to operate only within a certain frequency band. Thus, these devices transmit and/or receive signals in a specific frequency range and with predefined guard bands, and transmit mask. For example, commercial amplitude modulated (AM) radio broadcasts are transmitted in the 560-1600 KHz frequency spectrum, commercial frequency modulated (FM) radio broadcasts and television broadcasts are transmitted between approximately 50-700 MHz frequency spectrum while Bluetooth and WiFi devices operate in an unlicensed band between 2.4 to 2.4835 GHz. Those specific frequencies that the device uses to transmit and/or receive signals are typically referred to as a channel.
Throughout these specific bands of frequencies with which the devices use for their operation, there are portions of the frequency spectrum that remain unused. Additionally, the use of specific channels within the frequency band can alter depending on the time and/or location. For example, a television or radio broadcast may only transmit a signal on the channel at certain times of the day and at a certain power level. Similarly, the television or radio broadcast may only transmit a signal on the channel in one particular area or region yet while this spectrum in other areas or regions remains unused.
The Federal Communications Commission (FCC) in the United States, and its counterparts around the world, allocate radio spectrum across frequency channels of varying bandwidth. Various bands may cover, for example, AM radio, VH television, cellular phones, citizen's-band radio, pagers and so on. As more devices go wireless, an increasingly crowded amount of radio spectrum needs to be shared. Although the radio spectrum is almost entirely occupied, not all devices use portions of the radio spectrum at the same time or location. At certain times and location, a large percentage of the allocated spectrum may be sitting idle, even though it is officially accounted for. Regulatory authorities are beginning to permit usage of allocated spectrum on a secondary basis under certain strict constraints. For example, the FCC is beginning to permit the secondary usage of channels 21-51, also known as TV white space.
Cognitive radio is a term used to describe a suite of technologies with the potential to significantly alter the manner in which spectrum is utilized by future radio systems. A paradigm for wireless communication in which either a network or wireless device alters its transmission or reception parameters to avoid inference with licensed or unlicensed incumbent users, cognitive radio implements measures to avoid selecting an occupied frequency, so as to avoid interference that can possibly damage the incumbent device and/or reduce its signal reception quality. The alteration of parameters is based on active monitoring of several factors in the external and internal radio environment, such as radio frequency usage, user behavior and network state.
Cooperative spectrum sensing is a technique used to increase the probability of detection of primary users leading to reduced interference to the primary users by the cognitive radio network. However, cooperative sensing has certain drawbacks when individual nodes experience correlated fading or shadowing effects. Shadowing or long term fading refers to variation in received power due to large obstacles between the transmitter and the receiver.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. A class of ad-hoc networks called mesh networks, support multiple frequencies as well as multiple hops. As cognitive radio applications expand, it would be desirable to add cognitive radio compatibility powered by cooperative spectrum sensing within a multi-hop network, while keeping in mind that interference issues to the licensed users need to be addressed.
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